1. Field of the Invention
The present invention relates to a video signal processor that accurately removes cross-color noise, or cross-color noise and random noise, from the demodulated color difference components of a video signal of the National Television System Committee (NTSC) or Phase Alternation by Line (PAL) type in a video device such as a video display device or a video recording and reproducing device, avoiding color smear due to motion.
2. Description of the Related Art
Prior art relating to this type of video signal processing is disclosed in, for example, Japanese Patent Application Publication No. 2004-128936. FIG. 1 of that Publication shows a video signal processor that examines pixels at the same positions in two consecutive frames and detects motion from the frame-to-frame differences of the luminance values of the pixels. If no motion is detected, cross-color noise (crosstalk from the luminance component into the color components) is detected from frame-to-frame differences in the color difference signals, and if cross-color noise is present, it is removed by averaging the color difference signals of the current frame and a previous frame.
This method relies on the formulas listed in Table 1, which hold when the color difference component signals demodulated from an NTSC or a PAL television video signal do not vary over time, regardless of the presence or absence of cross-color noise.
TABLE 1Cross-color in Color Difference SignalsNTSCPALCurrent inputC′(n) = C(n) + ycc(n)C′(n) = C(n) + ycc(n)color differencesignal180° phase-offsetC′(n − 1) = C(n − 1) −C′(n − 2) = C(n − 2) −color differenceycc(n − 1)ycc(n − 2)signal
In this table, n represents the current frame, n−1 represents the immediately preceding frame, n−2 represents the second preceding frame, C represents the true color difference component, ycc represents a luminance component that crosses into the color difference component during luminance/chrominance (Y/C) separation (cross-color noise), and C′ represents the separated color difference signal.
When the input video data form a still picture, the following formulas hold for NTSC and PAL television signals.                NTSCC(n)=C(n−1)  (1)ycc(n)=ycc(n−1)  (2)        PALC(n)=C(n−2)  (3)ycc(n)=ycc(n−2)  (4)Accordingly, the result of averaging the current color difference signal C′(n) and the immediately preceding color difference signal C′(n−1) for NTSC, and averaging the current color difference signal C′(n) and the second preceding color difference signal C′(n−2) for PAL, is the true color difference component C(n), the cross-color component ycc being eliminated. This averaging process is independent of the value of ycc and also works when ycc=0; when there is no temporal variation (no motion), the averaged value is equal to the desired color difference signal C(n) regardless of the presence or absence of cross-color.        
A problem with the method disclosed in the above Patent Application Publication is that no motion is assumed to be present whenever there is no detectable luminance motion between two frames, even if motion is present in the color components. This can occur (a) if the only frame-to-frame change in the luminance signal occurs at high frequencies near the chrominance subcarrier frequency (fsc), which are removed from the luminance signal (and become a moving cross-color component) during Y/C separation, or (b) if the luminance signal remains substantially unchanged at all frequencies, but the color difference signals change from one frame to the next. If the averaging process is selected when color motion is present, unwanted color effects such as color smearing may appear.
A further problem is that in the PAL format, which has a four-frame color difference cycle in which the phase reverses once every two frames, the two frames that are averaged are separated by a two-frame interval, so to detect motion accurately, the luminance signal must be retained for the same two-frame interval; that is, a two-frame luminance signal memory is required instead of a one-frame memory.
Another problem is that although removal of cross-color noise by the averaging method also has some effect in reducing random noise, the effect is only a halving effect (−6 dB), so half the random noise tends to remain.
A more effective random noise reduction method that operates recursively is known, although it is not mentioned in the above Patent Application Publication. Recursive noise reduction to remove random noise could be applied separately from the averaging process that removes cross-color noise, but then two frame memories would be required.